A conventional shutter blade of the above-described structure will now be described by referring to FIGS. 3 and 4, where a shutter has a base plate 1 provided with an aperture 1a. Arms 2 and 3 are pivotally mounted to the plate 1 by shafts 4 and 5, respectively. A slit plate 6 having a slit for exposure is pivoted to the arms 2 and 3 by shafts 7 and 8, respectively. Similarly, a cover plate 9 is pivotally mounted to the arms 2 and 3 by shafts 10 and 11, respectively. The aperture 1a is covered by the slit plate 6 and the cover plate 9.
Only one of two similar shutter blades which are symmetrical with respect to a horizontal line is shown in FIGS. 3 and 4; for clarity, the other is not shown. In the condition shown in FIG. 3, the shutter blade is folded up and has moved away from the aperture 1a. In the condition shown in FIG. 4, the shutter blade is in a position to cover the aperture 1a.
When the shutter is tripped for exposure, the shutter blade is very quickly switched from the condition shown in FIG. 3 to the condition shown in FIG. 4 or vice versa. During this process, the arms 2 and 3 rotate to move the slit plate 6 and the cover plate 9 upwardly or downwardly while they are maintained in a horizontal posture.
Because of the inertia of various components and errors arising during the manufacturing process, the slit plate 6 may be inclined relative to the aperture 1a. Therefore, the plate 6 is not retained in the horizontal posture while moving. As a result, the exposure becomes non-uniform, i.e., the exposure time differs between the right portion and the left portion of the aperture. In recent years, it has been required to shorten the exposure time. With this trend, the width of the slit is reduced. Consequently, the effects of the inclination of the slit plate become more conspicuous.